Microwave devices

ABSTRACT

In a microwave device incorporating a component formed of dielectric material, and so designed that the response of the device is dependent on the permittivity of the said material, the component is formed of a ceramic material consisting of one or more alkaline earth metal zirconates, or zirconates and titanates, together with niobium pentoxide and/or tantalum pentoxide, the composition of the material being such that the atomic ratio of zirconium to titanium is not less than 80 : 20, that the total proportion of niobium pentoxide/tantalum pentoxide is in the range of 0.1 to 3.0 mole per cent of the total amount of the zirconate/titanate constituent, that it does not contain more than 10 mole per cent of barium titanate, and that the material will have, at microwave frequencies, permittivities in the range of 25 to 75, a substantially constant temperature coefficient of permittivity, which is preferably within the range from +50 to -100 p.p.m. per degree Centigrade, and a loss tangent not exceeding 0.001 at 20*C. The dielectric materials are advantageous for use, for example, as resonators for microwave bandpass filters, and as substrates for microwave integrated circuits. In the cases of some of the materials, the inclusion of niobium pentoxide/tantalum pentoxide reduces the microwave losses, as compared with similar materials without such additions.

United States Patent [191 Kell [54] MICROWAVE DEVICES [75] Inventor:

row, England [73] Assignee: The General Electric. Company Limited,London, England [22] Filed: Nov. 18, 1970 [21] Appl. N0.: 90,655

[30] Foreign Application Priority Data Dec. 11, 1969 Great Britain..60,579/69 [52] US. Cl. ..333/73 R, 333/73 S, 333/73 W,

333/83 R, 333/84 M [51] Int. Cl II0lp 3/08 ,'l-I0lp 7/06, H03h 7/08 [58]Field of Search.333/84, 83 R, 84 M, 73 S, 73 W [56] I References CitedUNITED STATES PATENTS 1 2,626,220 1/1953 Thurnauer et a1. ..l06/39 R3,534,286 10/1970 Holm et a1. ..l06/39 R 3,534,301 10/1970 Golembeski..333/84 M X FOREIGN PATENTS OR APPLICATIONS 755,860 8/1956 GreatBritain ..106/39 R 11/1966 Great Britain ..l06/39 R OTHER PUBLICATIONSPucel et al. Losses in Microstrip in IEE Transactions on MicrowaveTheory andTechniques V01 MTT 16 No. 6.June 1968. i

Robert Christopher Kell, South l-Iarn 3,713,051 51 Jan. 23, 1973 PrimaryExaminer-Herman Karl Szaalbach Assistant ExaminerMarvi n NussbaumAttorney-Kirschstein, Kirschstein, Ottinger & Frank [57] ABSTRACT In amicrowave device incorporating a component formed of dielectricmaterial, and so designed that the response of the device is dependenton the permittivity of the said material, the component is formed of aceramic material consisting of one or more alkaline earth metalzirconates, or zirconates and titanates,

. together with niobium pentoxide and/or tantalum pentoxide, thecomposition of the material being such that the atomic ratio ofzirconium to titanium is not less than 80 20, that the total proportionof niobium pentoxide/tantalum pentoxide is in the range of 0.1 to 3.0mole per cent of the total amount 01- the zirconate/titanateconstituent, that it does not contain -talum pentoxide reduces themicrowave losses, as

compared with similar materials without such additions.

4 Claims, 5 Drawing Figures MICROWAVE DEVICES This invention relates toelectrical devices of the kind designed for operation at microwavefrequencies, that is to say frequencies in the range of 400 MHz to 30GHz, for use for example in telecommunications equipment, andincorporating components formed of dielectric materials, wherein theresponse of the device is dependent upon the permittivity of thedielectric material.

United States Pat. Application Ser. No. 64,301, filed in the names ofRobert Christopher Kell, David Forbes Rendle and Eric Edward Riches onAug. 17, 1970, for Improvements in or relating to Microwave Devices, andassigned to the Assignee of the present application, relates tomicrowave devices of the aforesaid kind, in which the dielectriccomponent is formed of a ceramic dielectric material consisting of atleast one compound of the general formula A80 where A is a metal of thegroup consisting of barium, strontium and calcium and B is a metal ofthe group consisting of zirconium and titanium, the composition of thematerial being so chosen that the atomic ratio of zirconium to titaniumis in the-range of 80 20 to 100 0, that it does not include significantamounts of both barium and titanium and that the material will have, atmicrowave frequencies, permittivities in the range of 25 to 75, asubstantially constant temperature coefficient of permittivity, and aloss tangent not exceeding 0.005 at 20 C.

Microwave devices of the kind referred to in the aforesaid applicationinclude, for example, a microwave bandpass filter incorporating aresonator of dielectric material as specified, in replacement for themetal waveguide resonator incorporated in a conventional microwavefilter, and an integrated microwave circuit in which the dielectricmaterial is used to form the substrate carrying the conducting stripsconstituting the circuit elements.

Suitable dielectric materials for use in such devices, in accordancewith the aforesaid specification, include calcium zirconate, andcombinations of barium zirconate and strontium zirconate, bariumzirconate and calcium zirconate, strontium titanate and strontiumzirconate, and calcium titanate andcalcium zirconate. Whilst all thedielectric materials referred to above have loss tangents, at microwavefrequencies, not exceeding 0.005, and in some cases the loss tangentsare less than 0.001 at 20C, it is desirable that, for the applicationsreferred to, the microwave losses should be as low as possible, and someof the materials are less advantageous than others in this respect. Forexample, the barium strontium zirconates have microwave losses in excessof 0.001: one particular barium strontium zirconate containing bariumand strontium in the atomic ratio of S6 44, which is especiallyadvantageous for some microwave applications since its temperaturecoefficient of permittivity is near zero, would be even -more suitablefor use in these applications if its small proportion of niobiumpentoxide or tantalum pentoxide in the materials.

' Thus according to the present invention, in a microwave deviceincorporating a component formed of dielectric material, and so designedthat the response of the device is dependent upon the permittivity ofthe said material, the said component is formed of a ceram ic dielectricmaterial consisting of at least one compound of the general formula A130where A is a metal of the group consisting of barium, strontium andcalcium and B is a metal of the group consisting of titanium andzirconium, together with at least one oxide of the group consisting ofniobium pentoxide and tantalum pentoxide, the composition of thematerial being so chosen that the atomic ratio of zirconium to titaniumis in the range of 20 to 0, that the total proportion of niobiumpentoxide and tantalum pentoxide is in the range of 0.1 to 3.0 mole percent of the total amount of the compounds ABOQ, that the material doesnot include significant amounts of both barium and titanium and that itwill have, at microwave frequencies, permittivities in the range of 25to 75, a substantially constant temperature coefficient of permittivity,and a loss tangent not exceeding 0.001 at 20C.

As explained in the specification of Application No. 64,301, if thedielectric material contains a relatively large proportion of eitherbarium or titanium, there should not be a sufficient amount of the otherone of these elements present to make it possible for barium titanate tobe formed in a proportion which will cause the specified limits ofpermittivity and loss tangent of the material as a whole to be exceeded,and in particular the proportions of barium and titanium present shouldbe such that barium titanate does not constitute more than 10 mole percent of the material.

As also explained in the aforesaid application, the compound orcompounds ABO may be so chosen, and where two or more of such compoundsare present the relative proportions of the compounds may be soadjusted, that the dielectric material. as a whole has a temperaturecoefficient of permittivity of a desired positive or negative value, avalve within the range of +50 to l00 p.p.m. per degree Centigradeusually being preferred and in some cases the composition of thematerial being balanced to give a value of, or near, zero.

The dielectric material may consist of a single compound of the type A80having the requisite-properties, such as calcium zirconate, or of amixture or solid solution of two or more of such compounds, togetherwith a proportion of niobium pentoxide and/or tantalum pentoxide in therange specified. Since a material consisting of a single phase solidsolution is more readily reproducible than a material consisting of amixture of compounds, where two ABO compounds are present it is ingeneral preferred to employ combinations of compounds which form such asingle phase,

that is to say to use combinations of barium-barium,strontium-strontium, barium-strontium, or calcium-calcium compounds.Particularly preferred combinations and strontium titanate-zirconates inwhich the atomic ratio of titanium to zirconium is in the range of 2 98to 8 92. The calcium-containing materials tend to have increased lossesand variable temperature coefficients of permittivity in the presence ofmoisture: it may therefore be necessary to ensure that moisture isexcluded from these materials during use.

One example of a device in accordance with the invention is a microwavebandpass filter incorporating one or more dielectric resonators in theform of bars, cylinders or discs of dielectric material as specifiedabove: resonators of this kind are advantageous in comparison with theconventional metal waveguide resonators, since the use of a dielectricenables the size of the resonator to be reduced. In use, a ceramicdielectric resonator is usually placed within a metal screen, whichresults in a slight increase in the resonant frequency of the dielectricelement. Another type of device in which the aforesaid dielectricmaterials can be employed with advantage is an integrated microwavecircuit, the dielectric material being used to form the substratecarrying the circuit elements. The dielectric materials employed inaccordance with the invention are advantageous in this connection, ascompared with high density alumina which has hitherto been proposed forthis application, since they have higher permittivities, and lowertemperature coefficients of permittivity, than those of alumina.

The dielectric materials for use in the devices of the invention can beprepared by techniques conventionally employed for the production ofceramic dielectric materials of this type, that is to say by preparingan intimate mixture-of suitable powdered starting materials in therequired relative proportions, pressing the mixture, and heating thepressed compacts to effect reaction and sintering. If desired thematerials can be prepared from mixtures of the requisite pre-formedcompounds of the formula A80 together with niobium pentoxide and/ortantalum pentoxide, but preferably the ABO, compounds are prepared fromstarting mixtures comprising the constituent oxides and/or compounds,such as carbonates or hydroxides, which decompose on heating to give theoxides.

The niobium pentoxide and/or tantalum pentoxide maybe initiallyintroduced into the dielectric material either in the free state or inthe form of an alkaline earth metal niobate or tantalate of the generalformula MR O or M R O where M is barium, strontium or calcium and R isniobium or tantalum. In some cases, when the niobium and/or tantalum isinitially introduced as the free oxide, it might be possible, ordesirable, to reduce the content of zirconium or titanium in thedielectric material by an amount atomically equivalent to the amount ofniobium and/or tantalum introduced, the niobium/tantalum thus replacingpart of the zirconium/titanium in the dielectric composition.

A preferred procedure for preparing the dielectric components for use indevices in accordance with the invention, by which ceramic bodies ofdensity approaching the theoretical density, and hence having optimumpermittivity, can be obtained, includes the steps of isostaticallypressing the powdered starting mixture to form compacts of simpleshapes, such as rods, prefiring at a sufficiently high temperature toeffect partial sintering so as to form coherent bodies, then crushingthe prefired compacts to powder, die-pressing the powder to formcompacts of the desired shapes of the components to be produced, andfiring these compacts at a temperature higher than that employed for the5 prefiring step to convert them into dense, sintered,

ceramic bodies, the niobium pentoxide and/or tantalum pentoxide, in thefree state or in the form of compounds as aforesaid being eitherincluded in the initial starting mixture or added to a powdered prefiredmaterial consisting only of the desired compound or compounds A80 priorto the die-pressing and sintering steps. 7

The niobium pentoxide/tantalum pentoxide is thus incorporated in thedielectric material composition before or during the final sinteringprocess, and appears to go into solid solution in the A30 material.

The preparation and properties of some dielectric components for use indevices in accordance with the invention, together with the preparationand properties of components of similar materials without niobiumpentoxide or tantalum pentoxide, for comparison, will now be describedin the following specific examples.

EXAMPLE 1.

For the preparation of a disc (A) of barium strontium zirconate ofcomposition Ba Sr ZrQ- powdered barium carbonate, strontium carbonateand zirconium dioxide were mixed in the required relative proportionsand the powder mixture was milled with water in aporcelain ball mill for36 hours, then dried and compacted into rods under hydrostatic pressureof 7 tons per square inch, and the rods were prefired in air at l250Cfor 2 hours. The prefired rods were crushed in a disc mill, and theresulting powder was wet milled in a ball mill for 24 hours. The powderwas then dried, mixed with a solution of 2 wt. percent camphor in ether,and die-pressed under a pressure of 9 tons per square inch, to form adisc, which was finally sintered by firing in air at 1450C for 2 hours.

Two further discs (B, C) were prepared in the manner described above,with the addition of powdered niobium pentoxide, in amounts,respectively, of 0.25 and 1.0 mole per cent of the barium strontiumzirconate, to the prefired powder before the die-pressing and'sinteringsteps.

EXAMPLE 2.

A disc (A) of calcium zirconate was prepared by the method described inExample 1, using calcium carbonate and zirconium dioxide powders asstarting materials. Further discs (B, C) were prepared in the samemanner with the addition of, respectively, 0.25 and 1.0 mole per cent ofniobium pentoxide to the prefired powder.

EXAMPLE 3.

perature coefficient of permittivity is small, that of Some of theproperties of the materialsprepared as described in the above Examplesare given in the following Table. The properties which have beendetermined are the permittivity, loss tangent, and temperafrequencybeing determined for such discs 20 mm in diameter and 4 mm thick,resonated in the TE mode in a closely fitting waveguide reflectioncavity cut-off in the air regions.

TABLE pm Add d Properties at frequency 1.6 kHz. Properties at frequency5 gHz.

e Composition NbzOs, 10XTC C Permit- 10 Xloss IO XTCF Permit- 10 Xloss(A130 compound mol per C. tivity at tangent per C. tivity at tangentExample or compounds) percent error C. at 100 C. 20 C. at 20 C.

(A) fio-sa ro-ii os 0 0 38.1 11 -17 6 34, 7 I 18 Bn aSr ZrOs 0. -11 30.64 -14. 4 31. s 4. 7 1(C) BamstSmnZrOa 1.0 0 30.5 4 23. 7 32.3 4.4 CaZrOa0 +83 32. 0 2s. 0 7. 6 0. 25 +32 31. 1 72 17. 3 27. 2 5. 5 1. 0 +91 30.9 247 15. 7 27. 1 6. 3 0 0 36.1 3 -21.1 33.4 7.0 0. 25 0 37. 2 3 14.4=33. 4 6. 2 SrZmmssTlomsOa 1.0 +20 36. 1 3 -23. 7 33.3 4. 9

ture coefficient of capacitance (TCC) at audiofrequency (1.6 kHz), andthe permittivity, loss tangent, and temperature coefficient of resonantfrequency (TCF) at microwave frequency (5 GI-Iz). Audio frequencymeasurements were carried out, as well as microwave frequencymeasurements, because knowledge of the audio frequency properties of amaterial is of value in giving an indication of the pro- 2 perties thematerial will possess at microwave frequencies, and audio frequencymeasurements are more easily made.

The temperature coefficients of permittivity of the materials were notdetermined directly, but can readily be deduced from the temperaturecoefficient of capacitance, or from the temperature coefficient ofresonant frequency of a microwave cavity containing a disc of thematerial, which properties are more conveniently measured at-audiofrequency and microwave frequency respectively. Thus the temperaturecoefficient of permittivity is derived from the temperature coefficientof capacitance by subtracting from the latter the coefficient of thermalexpansion of the material, which for these ceramic materials in only 810 X 1O- /C, or is derived from the temperature coefficient of resonantfrequency by solving the resonator equations as given by S. B. Cohn andK. C. Kelly in an article published by the Institute of Electrical andElectronics Engineers, in the Transactions on Microwave Theory andTechniques, Volume 14 (1966), page 406. In practice, the importanttemperature coefficient for microwave applications is that of theresonant frequency (which can be measured) rather than that of thepermittivi ty (which must be calculated). The resonant frequency isrelated to E where E is the permittivity, and the temperaturecoefficient of resonant frequency is related to times the temperaturecoefficient of permittivity. It is therefore expected that if thetemresonant frequency will also be small, and if the temperaturecoefficient of permittivity is large, that of resonant frequency will belarge and of opposite sign.

For carrying out the measurements of the properties referred to, ataudio frequency, the major faces of the sintered discs of the materials,prepared as described above, were lapped to produce flat parallelsurfaces and silver paste was applied to these surfaces, dried at 120Cfor 12 hours and fired at 650C for one hour. The measurements ofmicrowave properties were carried out on non-metallized discs at afrequency close to 5 GHz, the temperature coefficients of resonant Theabove Table shows that the incorporation of niobium pentoxide in thebarium strontium zirconate material, while only slightly reducing theloss tangent at audio frequency, effects a considerable reduction in theloss tangent at microwave frequency, at the same time slightly reducingthe permittivity but not having a marked effect on the temperaturecoefficients of capacitance and resonant frequency. However, theaddition of niobium pentoxide appears to result in only a slightreduction of the loss tangent, at microwave frequency, in the cases ofstrontium zirconate-titanates and calcium zirconate, which materialshave lower microwave losses than barium strontium zirconates in theabsence of niobium pentoxide additions.

' The dielectric components in accordance with the invention, preparedas described in the above Examples, and listed in the Table, aresuitable for use as resonators for filter elements. Suitably shapedplates of the same materials, of thickness about 1 mm, can also be usedas substrates for integrated microwave circuits to be operated atfrequencies of l to 5 GI-Iz.

It will be appreciated that a device in accordance with the inventionmay incorporate more than one dielectric component as specified. Forexample, a microwave filter may comprise a number of dielectricresonators distributed along the axis of a waveguide used below itscut-off frequency.

Two specific microwave devices in accordance with the invention areshown in the accompanying drawings and will now be described by way ofexample. In the drawings, in which like parts in the different figuresare indicated by the same reference numerals,

FIG. 1 shows, in sectional elevation, a bandpass filter incorporatingfive dielectric resonators;

FIG. 2 is a sectional plan view of the filter shown in FIG. 1;

FIG. 3 is a transverse section of the filter shown in' FIGS. 1 and 2,drawn on the line IllIIII of FIG. 1;

FIG. 4 is a plan view'of a microstripline circuit on a dielectricsubstrate; and

FIG. 5 is a section drawn on the line V.V of FIG. 4.

Referring to FIGS. 1, 2 and 3 of the drawings, the relationship betweenwhich is indicated by the lines I-- I and IIII of FIG. 3 and IIIIII onFIG. 1, the device shown is a narrow band, high Q, filter designed tooperate at a frequency of 4 GiI-Iz, comprising five resonator discs 1formed of a dielectric material of a composition as specified inaccordance with the invention, suitably one of the Nb O containingmaterials ,listed in the foregoing Table, each disc having a diameter of20 mm, a thickness of 4 mm, and being adapted to resonate in the TEmode. The resonator discs are supported in a copper outer casing 2,suitably l4 cm long and 3.5 cm square in cross-section, by means of atube 3,cylindrical spacers 4 and rings 5, all formed of a low loss, lowpermittivity dielectric material, for example the material sold underthe Registered Trade Mark Rexolite the tube 3 being closed at both endsby copper caps 6. The resonator discs 1 have central holes 7 into whichare inserted rods 8 of the same dielectric material as the discsthemselves, and tuning screws 9 are inserted through the casing 2 tobear upon the rods 8 for adjusting the position of the rods in the holes7, in order to adjust the resonant frequency of the discs as required.

As shownin FIGS. 2 and 3, two 50 ohm Type-N connectors 10 are attachedto the casing 2, one at each end of the resonator disc assembly; coppercoupling strips 11, 12, for signal input and output respectively, aresoldered to the center pins 13 of the connectors, which pass throughapertures in the casing 2, and the copper strips are supported withinthe filter cavity by rings 14 of the same dielectric material as themembers 3, 4 and 5, referred to above.

FIGS. 4 and 5 of the drawings show a filter circuit in 50 ohmmicrostripline, 15, carried on a substrate 16 in the form of arectangular plate of a dielectric material of a composition as specifiedin accordance with the invention. The substrate may be, for example, mmlong, 12.5 mm wide and 0.8 mm thick and, as shown in FIG. 5, has acontinuous metal coating 17 on the face opposite to that on which thestripline circuit 15 is carried. Both the circuit 15 and the coating 17suitably consist of a layer of chromium covered with a layer of gold:these layers are formed on both sides of the dielectric plate byevaporating first chromium and then gold on to the faces of the plateand finally increasing the gold layer to the desired thickness byelectroplating; part of the coating is then removed from one face of theplate by photo-etching, to leave the desired circuit 15.

I claim:

I 1. A microwave bandpass filter comprising in combination v a. inputmeans,

b. output means, and

c. coupling means for coupling the input microwave signal energy to theoutput means,

d. said coupling means comprising at least one resonator in the form ofa body of dielectric material arranged to be subjected to the microwavesignal energyv so that the response of the bandpass filter depends onthe permittivity of the dielectric,

e. the said resonator body being formed of a ceramic dielectric materialconsisting of i. at least one compound of the general formula A whereinA. A is a metal of the group consisting of barium, strontium and calciumand B. B is a metal of the group consisting of zirconium and titanium,

ii. together with at leastone oxide of the group consisting of niobiumpentoxide and tantalum pentoxide, v

iii. the composition of the material being so chosen A that the atomicratio of zirconium to titanium isin the range of 80 20 to :0,

B. that the total proportion of niobium pentoxide and tantalum pentoxideis in the range of 0.1 to 3.0 mole per cent of the total amount of thecompounds ABO C. that if both barium and titanium are present theproportions thereof are such that barium titanate does not constitutemore than 10 mole per cent of the material, and

D. that the material will have at frequencies in the range of 400 MHz to30 GHz,

I. permittivities in the range of 25 to 75,

II. a temperature coefficient of permittivity which is substantiallyconstant with changes in temperature, and

III. a loss tangent not exceeding 0.001 at 20C, and

f. wherein the said resonator body has a hole formed therein, and g.there 18 provided a rod slidable in said hole and tuning means coupledto said rod to adjust the position of said rod in said hole whereby tovary the resonant frequency of said body.

2. A microwave bandpass filter according to claim 1, wherein the saidcompound A80 constituent of the dielectric material forming the saidresonator body consists of a barium strontium zirconate in which theatomic ratio of barium to strontium is in the range of 40 60 to 80 20. g

3'. A microwave bandpass filter according to claim 1, wherein the saidrod slidable in the hole of the resonator body is composed of the sameceramic dielectric material as the resonator body itself.

4. A microwave bandpass filter according to claim 1, which includes ahousing of low permittivity dielectric material and wherein saidresonator body of said ceramic dielectric material is in the form of adisc, said disc being disposed within said housing, and said input meansand said output means being disposed on said housing on opposite sidesof said disc.

2. A microwave bandpass filter according to claim 1, wherein the saidcompound ABO3 constituent of the dielectric material forming the saidresonator body consists of a barium strontium zirconate in which theatomic ratio of barium to strontium is in the range of 40 : 60 to 80 :20.
 3. A microwave bandpass filter according to claim 1, wherein thesaid rod slidable in the hole of the resonator body is composed of thesame ceramic dielectric material as the resonator body itself.
 4. Amicrowave bandpass filter according to claim 1, which includes a housingof low permittivity dielectric material and wherein said resonator bodyof said ceramic dielectric material is in the form of a disc, said discbeing disposed within said housing, and said input means and said outputmeans being disposed on said housing on opposite sides of said disc.